Exhaust gas treatment devices are used on automobiles to reduce pollution from engine exhaust gas emissions. Examples of common exhaust gas treatment devices include catalytic converters and diesel particulate traps.
A catalytic converter for treating exhaust gases of an automotive engine typically includes a housing, a fragile catalyst support structure for holding the catalyst that is used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen, and a mounting mat disposed between the outer surface of the fragile catalyst support structure and the inner surface of the housing to hold the fragile catalyst support structure within the housing during operation.
A diesel particulate trap for controlling pollution generated by diesel engines typically includes a housing, a fragile particulate filter or trap for collecting particulate from the diesel engine emissions, and a mounting mat that is disposed between the outer surface of the filter or trap and the inner surface of the housing to hold the fragile filter or trap structure within the housing during operation.
The fragile structure, either the catalyst support structure or diesel particulate filter, is contained within the housing with a space or gap between the exterior surface of the fragile structure and the inner surface of the housing. The fragile structure generally comprises a structure manufactured from a frangible material of a brittle, ceramic material such as aluminum oxide, aluminum titanate, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These materials provide a skeleton type of structure with a plurality of gas flow channels. These structures can be so fragile that small shock loads or stresses are often sufficient to crack or crush them. In order to protect the fragile structure from thermal and mechanical shock and other stresses encountered during normal operation of the device, as well as to provide thermal insulation and an effective gas seal, the mounting or support mat material is positioned within the gap between the fragile structure and the housing.
The mounting mat materials employed should be capable of satisfying any of a number of design or physical requirements set forth by the fragile structure manufacturers or the exhaust gas treatment device manufacturers. For example, the mounting mat material should be capable of exerting an effective residual holding pressure on the fragile structure, even when the exhaust gas treatment device has undergone wide temperature fluctuations, which causes significant expansion and contraction of the metal housing in relation to the fragile structure, which in turn causes significant compression and release cycles for the mounting mats over a period of time.
There are typically two types of mounting mats which may be used in exhaust gas treatment devices, namely, intumescent and non-intumescent mounting mats. Intumescent mounting mats contain materials which expand in response to the application of heat. The use of intumescent materials in mounting mats depends upon the intended application and the conditions under which the mounting mats are to be subjected. Non-intumescent mounting mats are substantially non-expanding. By “substantially non-expanding” is meant that the mat does not readily expand upon the application of heat as would be expected with intumescent mats. Of course, some expansion of the mat does occur based upon its coefficient of thermal expansion. The amount of expansion, however, is very insubstantial as compared to the expansion of intumescent mats.
All mounting mats are subject to edge erosion when incorporated within an exhaust gas treatment device. Hot emission gas generated by an internal combustion engine, passes through an exhaust pipe before entering the exhaust gas treatment device. In passing through the exhaust pipe, these emissions must enter an inlet region and exit through an outlet region of the exhaust gas treatment device. The leading and trailing edges of the mounting mat of the exhaust gas treatment device are exposed to these hot gases, which in turn causes, degradation of the mounting mat edges.
Both intumescent and non-intumescent mounting mats are not always resistant to hot gas edge erosion. Improper installation of the support mat or a lack of holding force of the mounting mat within the housing may result in support mat erosion. Moreover, depending on the particular application, utilizing a non-intumescent mounting mat within an exhaust gas treatment device may provide insufficient protection and support to the exhaust gas treatment device. With respect to intumescent mats, it is known to bind a distinct narrow strip of non-intumescent mat to the edge of an intumescent mat in order to provide edge protection. However, this is a relatively expensive and complex procedure as it involves physically attaching two separate components together, namely, the mounting mat and the strip of non-intumescent mat, to each other.
Various methods of edge protection exist for intumescent mats. High-temperature resistant cloths and rigidizing solutions have also been used in an attempt to mitigate the problems of edge erosion. The use of high-temperature resistant cloths, for example, entails wrapping the edges of the mat to improve the resistance to gas erosion. The cloth edge treatment protects the mat's leading and trailing edge. Because of the complexities involved with fitting a mounting mat wrapped with high-temperature resistant cloth into the housing, the use of separate high-temperature resistant cloths is not a highly accepted solution.
It is further known to add a high-temperature rigidizing solution to a mounting mat that is disposed between a catalyst support element and a metal housing. The high-temperature rigidizing solution material may be added either before or after the mounting mat is assembled within the metal shell. Rigidizing solutions, while very effective, must be applied by the end user during the canning operation and are messy products to apply to the mounting mat. Furthermore, application of a rigidizing solution to the mounting mat prior to its assembly around the fragile structure is impossible, because the solution dries very hard and renders the mounting mat too stiff to be wrapped around the fragile catalyst support structure.